1. Field of the Invention
The present invention generally relates to a color cathode ray tube utilizing an in-line electron gun assembly and, more particularly, to a deflection yoke assembly used in the in-line color cathode ray tube.
2. Description of the Prior Art
The prior art in-line color cathode ray tube, that is, the cathode ray tube of a type wherein three electron guns are arranged in a line generally parallel to the direction of sweep of electron beams across the phosphor deposited screen, is schematically illustrated in longitudinal sectional representation in FIG. 6. As shown in FIG. 6, the cathode ray tube includes a highly evacuated envelope generally identified by 1, which envelope 1 comprises a funnel section 1B generally flared in one direction and having reduced and enlarged diameter ends at its opposite ends. A faceplate 1A is sealed to the enlarged diameter end of the funnel section 1B and a phosphor deposited screen 1b is formed on an inner surface thereof in the form of a pattern of triads of phosphor stripes. A generally cylindrical neck section 1C continues from the reduced diameter end of the funnel section 1B in a direction away from the faceplate 1A and an in-line electron gun assembly 2 is accommodated therein which includes three electron guns for the emission of electron beams 2B, 2G and 2R of different elemental colors, for example, blue, green and red. The envelope 1 also comprises a finely Perforated shadow mask 4 having a multiplicity of minute apertures for the selective passage of the electron beams 2B, 2G and 2R emitted from the respective electron guns of the electron gun assembly 2.
A deflection yoke assembly generally identified by 3 is mounted exteriorly on the highly evacuated envelope 1 at a location adjacent the boundary between the funnel section 1B and the neck section 1C. This deflection yoke assembly 3 comprises a pair of generally saddle-type horizontal deflection coils and a pair of generally toroidal vertical deflection coils both housed within a coil separator 3A of a shape having a radially outwardly flared front portion adjacent the reduced diameter end of the funnel section 1B, a generally conical portion and a radially outwardly flared rear portion adjacent the neck section 1C.
The color cathode ray tube of the above described construction operates in the following manner.
The electron beams 2B, 2G and 2R of different colors emitted from the electron gun assembly 2 sweep across the phosphor deposited screen 1b from left to right and from top to bottom by the action of the horizontal deflection magnetic field and the vertical deflection magnetic field developed respectively by the horizontal deflection coils and the vertical deflection coils in the deflection yoke assembly 3. After having been deflected about the center of deflection and prior to the electron beams 2B, 2G and 2R impinging upon the phosphor deposited screen 1b, the electron beams 2B, 2G and 2R of different colors pass through the minute apertures in the perforated shadow mask 4 and then impinge upon corresponding phosphor deposits on the phosphor deposited screen 1b to excite such corresponding phosphor deposits to illuminate to thereby form a color image. The impingement of the electron beams 2B, 2G and 2R upon the phosphor deposited screen 1b to excite the corresponding phosphor deposits is well known as a landing.
In most conventional color cathode ray tubes, it is quite usual that the radius of curvature of the phosphor deposited screen 1b is greater than the distance between the center of deflection of the electron beams and the center of the phosphor deposited screen 1b in alignment with the longitudinal axis of the evacuated envelope 1 and, therefore, the distance from the center of deflection to the. phosphor deposited screen 1b progressively increases with increase of the distance from the center of the phosphor deposited screen 1b to the perimeter of the phosphor deposited screen 1b. In other words, the center of curvature of the phosphor deposited screen 1b is not at the center of deflection of the electron beams. In such type of color cathode ray tube, where the deflecting magnetic fields developed by the deflection yoke 3 in horizontal and vertical directions are uniform and the color electron beams 2B, 2G and 2R are deflected by these uniform deflecting magnetic fields, the color rasters produced on the phosphor deposited screen 1b by the intermediate electron beam 2G and the side electron beams 2B and 2R on respective sides of the intermediate electron beam 2G do not exactly match with each other, particularly at a peripheral portion of the phosphor deposited screen 1b as shown in FIG. 7, thus creating a condition known as a dynamic misconvergence. It is to be noted that, in FIGS. 6 and 7, the direction parallel to the longitudinal axis of the evacuated envelope 1 is expressed by Z, and horizontal and vertical directions perpendicular to the direction Z are expressed respectively by X and y, all of these directions X, y and Z being as viewed on the phosphor deposited screen 1b.
In the color cathode ray tube tending to exhibit the dynamic misconvergence, if the distribution of the deflection magnetic fields developed by the deflection yoke assembly 3 is so designed and so chosen that the horizontal deflecting magnetic field can produce such a pincushion distortion as shown in FIG. 8 while the vertical deflecting magnetic field can produce such a barrel distortion as shown in FIG. 9, the side electron beams 2B and 2R can be converged at respective locations on the phosphor deposited screen 1b as shown in FIG. 10. At this time, although a distortion caused by the coma aberration renders the color raster produced by the center electron beam 2G to be somewhat undersized as compared with the color raster produced by each of the side electron beams 2B and 2R, the difference in size of the color rasters can be compensated for if the magnetic field leaking from a neck region of the deflection yoke assembly 3 is controlled by the use of a magnetic field controlling element directed to each electron beam so as to render the center electron beam 2G and the side electron beams 2B and 2R to substantially coincide with each other on the phosphor deposited screen 1b.
On the other hand, the raster distortion depends on a distribution of deflection magnetic fields. Specifically, top and bottom pincushion distortions PQ1 and left and right pincushion distortions PQ2 shown in FIG. 7 as appearing on the phosphor deposited screen 1b are mainly attributable to the distribution of the horizontal deflection magnetic field and the distribution of the vertical deflection magnetic field, respectively, and can be minimized as the deflection magnetic fields are so developed as to produce the pin-cushion distortions. Accordingly, if in order to compensate for the misconvergence the horizontal deflection magnetic field is strongly distributed in a pattern similar to the pincushion distortion as shown in FIG. 8 and the vertical deflection magnetic field is strongly distributed in a pattern similar to the barrel distortion as shown in FIG. 9, the top and bottom pincushion distortions PQ1 appearing on the phosphor deposited screen 1b can be substantially eliminated, but the left and right pincushion distortions PQ2 will be enhanced.
In view of the foregoing, it is a general practice to employ the system wherein, as shown in FIG. 11, a portion of the deflection yoke assembly 3 facing towards the phosphor deposited screen 1b is so tailored as to develop a pincushion magnetic field while the remaining portion of the deflection yoke assembly 3 is so tailored as to develop a substantially intensified barrel-shaped magnetic field, thereby rendering the total amount of the vertical deflection magnetic field, which would act on the electron beams 2B, 2G and 2R, to represent a generally barrel-shaped field. With this system, no correction of the dynamic convergence is required and, at the same time, no dynamic correction of the raster distortions is also required.
A technique to render the correction of the dynamic convergence and the raster distortions to be unnecessary is disclosed in any one of the U.S. Pat. Nos. 4,143,345, 4,246,560 and 4,257,023, issued Mar. 6, 1979, Jan. 20, 1981, and Mar. 17, 1981, respectively According to these United States Patents, the use has been made of magnetic pieces arranged at the center portion of the yoke length or in the vicinity of the vertical deflection coils.
However, if the curvature of the phosphor deposited screen 1b of the color cathode ray tube is small or the phosphor deposited screen 1b of the color cathode ray tube is of a shape composed of a plurality of curvatures, both of the complete convergence and the elimination of the raster distortions by relying only on the distribution of the magnetic fields developed by the deflection coils or on a combination of the distribution of the magnetic fields developed by the deflection coils with the magnetic pieces cannot be accomplished without difficulty.
FIG. 12 illustrates the conventional deflection yoke assembly, as viewed from rear, which has been contemplated to accomplish both of the convergence and the correction of the raster distortions. In FIG. 12, reference numeral 5 represents a pair of permanent magnets mounted on the radially outwardly flared front portion 3a of the coil separator 3A of the deflection yoke assembly 3 in alignment with a vertical axis y perpendicular to the longitudinal axis of the evacuated envelope, which magnets 5 are operable to correct top and bottom raster distortions and also to correct both of the convergence and the raster distortions by means of the distribution of the magnetic fields developed by the deflection coils.
In the conventional deflection yoke assembly of the above described construction, where the color cathode ray tube is so designed and so structured that the curvature of the phosphor deposited screen in the horizontal direction and that in the vertical direction can be expressed by secondary and fourth-order functions, respectively, the use of the permanent magnets capable of emanating a high magnetic force is required, and, even though raster distortions at top and bottom of the phosphor deposited screen 1b as shown in FIG. 13 could be successfully eliminated by the employment of the permanent magnets of high magnetic force, gull distortions PQ3 appearing at a portion of the phosphor deposited screen 1b generally intermediate between the center and the top or the bottom as shown in FIG. 13 cannot be successfully eliminated. Also, since the permanent magnets of high magnetic force are employed, any variation in gaussing force of the permanent magnets tends to adversely affect the raster distortions and the landing characteristic of the electron beams.